PNNL

As the weather warms and recreational areas reopen after months-long COVID-19 closures, many of us are anxious to take to the great outdoors.

The forests of the Northwest are especially alluring, offering wonderful opportunities for day hikes and summer camping trips.

If you are a long-time fan of the forests and think you've noticed a change over the years, you're right. Researchers at the Department of Energy's Pacific Northwest National Laboratory are studying the impacts of human activity and climate change on our wooded retreats.

From the forests on either side of the Cascades to old-growth forests around the world, researchers are seeing a shift toward younger and shorter trees. This trend was recently discussed in a paper in the prestigious journal Science and is cause for concern.

PNNL Earth scientists, along with collaborators from more than a dozen institutions, analyzed more than 160 previous studies to understand how forests are changing.

By combining advanced Earth system models and remote sensing data on underlying processes and impacts in vegetation, they sought to determine what is happening to our forests and why.

Around the world, they found at least one-third of old growth forests were lost between 1900 and 2015. In North America, as well as Europe and the Amazon, tree mortality has doubled in just the last 40 years.

Delving into this data showed that rising temperatures play the most significant role in the loss of old growth forests. Warmer conditions mean less moisture in the soil and leaves, even if rainfalls remain the same.

In addition, land-use changes, timber harvests and increasing carbon dioxide, as well as wildfires, drought, disease and insect infestations, play their part in the forests' decline.

While forests can often respond to these disturbances individually with only localized impacts, the combined effects of these factors over time are changing the demographics of our forests.

Scientists studied the distribution of seedlings just getting their start, trees that were growing strong and those no longer able to reproduce — all of which contribute to a healthy, thriving forest.

When old growth forests are disturbed, it upsets the circle of life. Mortality rates go up, the new and strong trees cannot compensate for what was lost, and the average age and height of the forest declines.

Furthermore, as old growth trees struggle to survive, they yield to species that are better adapted to the changing conditions.

Sometimes these new species begin dominating the younger, growing population, altering the makeup of forests in an entire region.

Looking to the future, researchers predict that the factors threatening old growth forests will accelerate. For example, rising temperatures and increased carbon dioxide bring about more frequent and severe wildfires and droughts that keep trees from growing tall and shorten their lifespan.

As a result, we are seeing new ecosystems emerge with younger, smaller trees that store less carbon and offer less biodiversity than their old-growth counterparts.

Not only would these forests be potentially less enticing for those seeking adventure, they also offer the planet fewer benefits when it comes to mitigating climate change. This results in a downward spiral of forest health.

John Muir, who is known as the father of America's national parks, once said, "The clearest way into the Universe is through a forest wilderness."

At PNNL, researchers are uncovering the secrets of trees and the underlying processes affecting them to get a better view of the future of our forests and the world around us.

The knowledge they gain may help inform forest managers and natural resource planners who seek to protect the forests, not only as places for an enjoyable getaway, but for the benefits they bring the Earth.

Related videos

• Global Changes Leading to Shorter, Younger Trees

Steven Ashby, director of Pacific Northwest National Laboratory, writes this column monthly. His other columns and opinion pieces are available here.